1. Field
This disclosure generally relates to lighting devices that employ active light sources, in particular, phosphor-based solid state lights.
2. Description of the Related Art
Lighting devices exist in a broad range of designs suitable for various uses. Some lighting devices illuminate interior spaces, while others illuminate exterior spaces. Some lighting devices are used to provide information, for example, forming part of or all of a display panel. Active lighting sources take a variety of forms, for example incandescent lamps, high-intensity discharge (HID) lamps (e.g., mercury vapor lamps, high-pressure sodium lamps, metal halide lamps), and solid-state light sources for instance light emitting diodes (LEDs).
Lighting devices have a number of defining characteristics, including intensity (e.g., lumens), focus or dispersion, and temperature of the emitted light. For light sources that emit light by thermal radiation (e.g., incandescent filament), the color temperature (CT) of the light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. Light sources that emit light by processes other than thermal radiation (e.g., solid state light sources) do not follow the form of a black-body spectrum. These light sources are assigned various correlated color temperatures (CCT) to indicate, to human color perception, the color temperature that most closely matches the light emitted.
Achieving desired lighting typically requires selecting suitable light sources, lenses, reflectors and/or housings based at least in part on the defining characteristics, the environment in which the lighting device will be used, and the desired level of performance.
LEDs are becoming increasingly popular due to their high energy efficiency, robustness, and long life performance. Typically, practical LEDs are capable of emitting light in a relatively narrow band. Since white light is often desirable, solid-state lighting systems typically employ “white” LEDs. These “white” LEDs may be manufactured by placing a phosphor layer either directly on a blue emitting LED die or onto a lens or window through which an LED will emit light. The phosphor layer is typically designed to convert radiation in the 440 to 480 nanometer wavelength range (mostly blue light) into a wider spectrum consisting of longer visible wavelengths that, when added to residual blue light, will appear as a pleasing white light. A variety of white LEDs are commercially available from a variety of manufacturers. Commercially available white LEDs range from “cool” white with a CCT of approximately 6000 Kelvin to “warm” white with a CCT of approximately 3000K.
In addition to the performance parameters described above, lighting of homes, offices and other areas often has esthetic concerns that are as important as the amount of illumination produced by the lighting system. Unlike an ideal black body radiator or natural daylight, solid-state lighting systems do not produce light that has a smooth and continuous spectral power distribution, despite the appearance of “white” light. FIG. 1 shows the spectral power distribution of a number of LED lights ranging from cool to warm, all of which show strong emissions in the blue and yellow regions.
It is known that phosphor-coated white LEDs permit some blue light to escape conversion by the phosphor, which light contributes to the blue spikes shown in FIG. 1. The blue light differs from natural light and also may appear harsh or otherwise unpleasing. In addition, other esthetic concerns often favor an emission spectrum that has more red and green wavelengths than would come from a true black body radiator. This type of light enhances the colors and color contrasts of furnishings and décor.
Although red and green light can be added to white LEDs to provide a more pleasing spectrum, this method may result in significant added cost for the extra LEDs and drive electronics, while the blue wavelength spike in the output spectrum remains.
Absorption filtered lamps, such as the General Electric's REVEAL® light bulbs) typically incorporate a filter element, such as neodymium, into the glass bulb to filter out the dull yellow light produced by the incandescent filament, thereby enhancing the appearance of the more vibrant light such as red. The addition of such a filter, however, causes a significant loss of light output, leading to a very low efficiency. For example, a REVEAL® 60 W bulb has a Lumens/Watt rating of only 11. Although an LED lamp may have a rating of 65 to 100 L/W, it can be expected that adding absorption filters would similarly reduce the efficiency as well as the light output, because the undesirable light is filtered and dissipated as heat. The heat added to the system from the absorptive filter may also contribute to lowering the life expectancy of the LED.
Adjusting the phosphor formulation of white LED lamps is also inadequate in providing the desired pleasing light in an LED, due to the wideband nature of the phosphor's emission spectrum. In other words, a narrow band of wavelengths typically cannot be removed from the white LED output spectrum by adjusting the phosphor formulation.
U.S. Published Application No. 20110175518, which is co-owned by the assignee of the subject application, describes various methods and apparatuses that can minimize undesirable blue light without the losses associated with absorption filtering.